The present invention relates to image forming apparatuses using the electro-photographic method, such as copiers, printers, facsimile machines and apparatuses combining these functions, and in particular, relates to a technique to improve durability of these apparatuses by preventing filming on the photosensitive drum or on the intermediate image transfer belt, or damage to the edge of the cleaning blade.
The blade cleaning method is known which is a cleaning method used in image forming apparatuses configured to form a toner image on a photosensitive drum employing various means for charging, exposure, and development around the drum, then either directly transferring the toner image onto the recording material (sheet of paper), or temporarily transferring the toner images from plural photosensitive drums onto an intermediate image transfer belt and then conducting a secondary transfer of the toner image on the belt onto a recording material. In order to remove any residual toner remaining on the photosensitive drums or on the intermediate image transfer belt (both of which are collectively called an image carrier) after the images have been transferred, commonly arranged is a cleaning blade made of an elastic material such as urethane which presses against the image carrier in the direction counter to the direction of movement of the image carrier.
Recently, increased printing rate and higher durability are required of an image forming apparatuses, and image defects have become problems, such as image striations and unevenness caused by filming of the photosensitive drum or the intermediate image transfer belt, as well as cleaning defects caused by wear or damage to the edge of the cleaning blade. Filming is caused by adhesion of lubricants such as calcium stearate (CaSt) and zinc stearate (ZnSt), or wax included in the toner onto the image carrier, and the electrical image transfer field of the adhered portion is weakened because the surface resistance there becomes higher. Further, because the adhesion force between the toner and the intermediate image transfer belt becomes stronger on the adhered portion, it becomes hard to remove the toner from the intermediate image transfer belt. For these reasons, the decrease of transferring performance on the adhered portions causes striation or unevenness of the image. Further, portions where filming appears raise their frictional drag and, hence the edge of the cleaning blade is scratched and bent on the surface of the intermediate image transfer belt, resulting in damage to the cleaning blade edge.
Lubricants are added into the toner to improve smoothness of the cleaning blade, and wax is added into the toner to improve separability of the recording material after the fixing process.
There is a tendency that more filming or damage occurs on the edge of the cleaning blade on intermediate image transfer belts than on photosensitive drums. The reason is considered to be because the surface of the intermediate image transfer belt is harder than that of the photosensitive drum and is hard to shave off. This condition makes it easier for materials such as the toner to adhere and accumulate on the surface of the intermediate image transfer belt.
As a countermeasure against these problems, a well-known technique to eliminate filming is to polish the image carrier.
Another well-known technique is to improve lubrication or polishing effect between the image carrier and the cleaning blade which reduces frictional drag by forming a toner band in areas other than the image area, which supplies adequate toner to the edge of the cleaning blade during non-imaging operation.
Further, to prevent bending of the cleaning blade by reducing frictional factors between the cleaning blade and the intermediate image transfer belt, there is an image forming apparatus which is structured to scrape off the residual toner adhering to the intermediate image transfer belt, and stores a part of the residual toner in a storage portion and further scoops up the toner for re-adhesion to the intermediate image transfer belt with a brush roller (for example, see Patent Document 1).
[Patent Document 1] Official Gazette of Japanese Patent Tokkaihei No. 11-38778
The image carrier may be polished with emery paper, non-woven fabric including abrasive particles or a lapping film by contact of these materials with the rotating image carrier. However, problems of the emery paper or the non-woven fabric include abrasive particles easily causing damage to the image carrier, and further lapping film which has a fine mesh easily becomes clogged and the polishing effect does not last long even though it is very effective for a short time.
When a toner band is formed on an area other than the image area on the intermediate image transfer belt, another problem is that if the toner band is formed between successive sheets of paper, the toner band adheres to the transfer roller and stains the back of the sheet. A structure considered to be effective is one which retracts the transfer roller while the toner band passes near the roller to prevent staining, however, in case of high speed machines, no enough time can be secured for the retraction, and therefore, forming the toner band causes adverse effects during the normal image forming operation, and further the number of the finished copies per unit time decreases, and still further consumption of toner increases.
Yet further, according to the image forming apparatus described in Patent Document 1, the residual toner is re-supplied to the intermediate image transfer belt. Unless the contacting load of the cleaning blade against the intermediate image transfer belt is increased, a problem occurs that the residual toner slips under the edge of the cleaning blade, however if the contacting load is increased another problem occurs that the edge is easily worn.